Imo just get the game made to be compatible with Steam. I know a few Mac gamers and they all us bootcamp to play most of their games. Sooo just make it for windows and Steam and if it becomes popular enough convert it over to Mac...just don't expect to make a substantial profit converting to Mac
http://store.steampowered.com/hwsurvey
Not everybody uses/likes Steam, I am not a fan of it myself, and would much rather not have to use it...
As far as I know, Steam is just a DRM, and doesn't so anything to enhance the game's compatibility. (Just have a look at the PC port of Killer is Dead...)
Besides, isn't there a Mac version of WINE? That's a really good emulator, used in Linux, which play Windows games almost as good as on Windows, and in some cases better!